string str = "";

// loop though an array and create a string
// delimited with commas
foreach (string s in fieldNames)
{
    str += s;

    // if this not the last item in the array
    // append a comma to end of the string.
    if (s != fieldNames[fieldNames.Length - 1])
    {
        str += ", ";
    }
}

This can be useful for string preparations.

Merging a String Array to a Comma delimited String (foreach)

array

C#